Increasing Triplet Subsequence

Problem: Increasing Triplet Subsequence

At first, I consider to use a triplet to store the subsequence. If we find a smaller number which is good to improve the possibility of a result, we can do swapping. One idea is that we can do swapping if there is one number currently in the triplet or store the smaller number in a backup if there are currently two numbers in the triplet.

Code in Python:

class Solution(object):
    def increasingTriplet(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        if len(nums) < 3: return False

        triplet = []
        _triplet = []
        for i in xrange(len(nums)):
            if len(triplet) == 0:
                triplet.append(nums[i])
            elif len(triplet) == 1:
                if nums[i] > triplet[0]: triplet.append(nums[i])
                else: triplet[0] = nums[i]
            else:
                if triplet[1] < nums[i]: return True
                elif nums[i] <= triplet[1] and nums[i] > triplet[0]: triplet[1] = nums[i]
                else:
                    while _triplet and nums[i] <= _triplet[-1]:
                        _triplet.pop()
                    _triplet.append(nums[i])
                    if len(_triplet) == 2: triplet = _triplet
        return False

However, if we are storing [1,2] in the triplet and meet with -1 for example, we can just swap 1 with -1 and don't interfere with the result. So there's a much better solution here.